1. Field of the Invention
The present invention relates to a process and a device for electromagnetic guidance, applied in particular to the tracking of targets. It applies in particular in respect of the guidance of any number of missiles or intelligent munitions, for example in the tracking of targets.
2. Description of the Background Art
The guidance of missiles and intelligent munitions requires a precision which depends on the nature of the target, as well as on the size and principle of operation of the associated warhead.
In the case of short and very short range tactical missiles (between 5 and 12 km range) which alone can be guided remotely, the longest ranges requiring auto-guidance modes, the precision required for the guidance can be summarized along the following lines.
Depending on the type of warhead the guidance precision must allow this warhead to fulfill its role; this precision can vary between 0.1 meters and 5 meters. The greatest precision applies to small missiles using the kinetic effect of a penetrator against targets of very small dimension (drones or tactical missiles), the lowest precision is applicable to missiles using a warhead with a charge of several kilograms, associated with a proximity fuse against lightly armored or unarmored objectives.
In remote guidance, the system relies on one or more remotely sited sensors, generally sited on the launch platform, which undertake the location of the target and of the missile or missiles (or projectiles) launched against this target.
Given the precision demands cited above, it follows that the angular precision of these sensors cannot be less than this.
These sensors using an electromagnetic means (radar or optical) have a diffraction limited resolving power. If one sets a reasonable order of magnitude for the diameter of the aperture of these sensors, namely 1 meter for radar antennas and 0.1 meters for visible or infrared optical ones, it is noted that onward of the infrared IV band (8-12 microns wavelength) the precision of location must be better than the diffraction spot. Whereas the gain in precision with respect to the diffraction spot is modest for infrared, it is considerable for the radar bands.
In general, such a specification regarding precision is not compatible with the precision of mechanical aiming of a sensor installed on a military platform, and at the very least the system will require stabilization of the line of sight.
Moreover, when the sensor serving to track the target is not the same as the sensor serving to locate the missile or missiles, a serious problem arises of collimating these sensors which is almost unachievable for the severest demands of precision.
An elegant way of solving this problem is to use a single sensor for target and missile location and to take the difference of the measurements according to the principle of xe2x80x9cdouble weighingxe2x80x9d which is well known to physicists. This principle has been dubbed xe2x80x9cdifferential deviometryxe2x80x9d, and has the merit of getting rid of most instrument errors such as shifting of the zero and all errors of aiming within the specification regarding precision of location of the missile with respect to the target.
Certain instrument errors such as errors of gain or of slope of the angle measurements which might not be identical for the target measurement and missile measurement are deterministic and can be compensated for to the extent that they may be known.
The specification then still contains errors such as the terms which are not absolutely deterministic, like the errors related to the noise of the system which among other things includes noise of thermal origin and noise related to the propagation environment for the electromagnetic waves employed.
When one wishes to operate relatively independently of the conditions of illumination and visibility, one is then led to choose a sensor of radar type.
The known solution of this type is broadly applied to tactical missile systems, in particular anti-aircraft systems.
In these systems, a tracking radar is furnished with several elevation and bearing measurement pathways which, in shared time, undertake the position measurements for the target and one and sometimes two missiles which are constrained to remain within the field of vision of the sensor generally of the order of magnitude of one degree of angle. For this reason, the missile or missiles remain substantially aligned with the moving line joining the radar sensor and the target, hence the name alignment guidance for this mode of operation.
To the extent that the position of the missile is remote with respect to the guidance platform, a transmission link (remote control) is necessary in order to convey the position thereof or more generally trajectory correction commands to the missile.
The locating of a missile and a fortiori of intelligent munitions of more modest size, requires that the radar signature of this missile or of these munitions be strengthened by means of a radar xe2x80x9cresponderxe2x80x9d which is appended to the necessary remote control receiver. It is appreciated that such an assembly is of a complexity such as to severely burden the weight and cost specifications of missiles to which they apply.
A simpler solution has been applied, especially in optical mode, in which the missile or missiles locate themselves with respect to a theoretical guidance axis associated with a director beam.
Hence the name beam navigation for this type of guidance, also known as xe2x80x9cBeam Riderxe2x80x9d. To do this, a spatially modulated beam projector is locked mechanically onto the detection axis of a tracking sensor.
A special receiver on board the missile decodes the modulation of the beam and determines its position with respect to the theoretical guidance axis and deduces any corrective maneuvers therefrom.
The space coding method employed is in general a two-dimensional scan of a pencil of light or of a double scan in elevation and bearing of two pencils having a narrow shape in one plane and fanning out in the other. Both methods as well as their numerous variants have the common denominator of being limited in terms of precision by the size of the diffraction spot and by the precision of the collimation of the beam projector and of the tracking sensor as well as of the precision of the mechanical tracking.
This precision can be slightly improved at the cost of greater complexity:
If the conditions of steadiness of the propagation so permit, the measurement precision can be sharpened by interpolating between several positions of the beam and the measurement precision can be lowered slightly below the defraction spot without being able to expect much better than a quarter or a half of the latter. In respect of the collimation errors, they may in principle be measured and compensated at the cost of a complex autocollimation system.
The tracking errors, except when the tracking loop is closed up manually via a human operator, are determined by means of an automatic tracking device; it is therefore possible in theory to carry out the inverse correction in the coding of the beam so that the measurement made by the missile has as origin the position of the target in the observation field and not a fixed mechanical reference.
However, owing to their cost and complexity, these refinements are rarely encountered in practice, thereby limiting the known use of beam navigation to very short range systems (at most 5 to 6 km) and using optical wavelengths, in general that of a laser operating in the visible or the near infrared (wavelength of around 1 micron). Furthermore, performance is dependent on the conditions of illumination and visibility, in a manner inherent to any optical system.
The beating beam systems described above are not able to be extended to radar wavelengths, since for practical antenna dimensions in the missiles involved, they do not permit the required precision.
To operate beam guidance in the domain of radar, consideration has been given to the use of the principle known by the name of conical scanning, this in fact being a variant of the principles above, mechanical scanning being obtained by rotating the radiation pattern of an antenna around an axis not pointing along its axis of revolution.
As a result, the gain in any direction other than that of the axis of rotation is modulated in a time periodic manner with a phase and an amplitude which are directly related to the polar coordinates of this direction in the field of observation.
For a radar, it is thus possible simultaneously to modulate the gain on transmission and on reception, this being the simplest solution, although in principle the modulation of a single pathway is sufficient.
If the transmission pathway is modulated, the modulation is easily detected by a countermeasures receiver, and a jamming transmission modulated at the same frequency will inevitably cause tracking to be lost, this making such a device very sensitive to jamming by a jammer even if the latter is carried by the target. Modulation on reception alone is a little less fragile but cannot be used for beam guidance, since, the transmission no longer being modulated, the missile cannot self-locate.
The technique of conical scanning has been abandoned for radars because of its sensitivity to jamming, and also to the other causes of amplitude fluctuation of the signal received, such as the unsteady nature of the propagation conditions in relation to the necessarily fairly slow mechanical scanning rates.
An aim of the invention is to alleviate the aforesaid drawbacks, by permitting in particular the guidance of craft with respect to a radar beam without risks of jamming. Accordingly, the subject of the invention is a process for the electromagnetic guidance of at least one craft, characterized in that the guidance is carried out with respect to the axis of the beam of a monopulse antenna, by coding in space of the beam. The subject of the invention is also a device for implementing the above process.
The main advantages of the invention are that it allows guidance of craft with respect to the antenna beam while undertaking target position measurements, that it allows very simple embodying of the receivers, that it allows the guidance of very crude or very small craft, that it can be easily combined with other means of guidance such as optical means or seekers to obtain multimode guidance, doing so without overly adding to the complexity of the system, that it allows very good precision of the craft position measurements, that it allows information to be sent simultaneously to the various craft and that it is economical.